The invention relates generally to methods and apparatus for improving the performance of a mass storage system.
Disk drive systems have grown enormously in both size and sophistication in recent years. These systems can typically include many large disk drive units controlled by a complex, multi-tasking, disk drive controller such as the EMC Symmetrix disk drive controller (a product of EMC Corporation, Hopkinton, Mass.). A large scale disk drive system can typically receive commands from a number of host computers and can control a number of disk drive mass storage devices, each mass storage device capable of storing in excess of several gigabytes of data. Each mass storage device can be representative of one or more logical volumes. Alternatively, a physical disk drive element may represent several of the logical volumes. The EMC Symmetrix data storage system is configured to include as many 4096 logical volumes.
Each logical volume has an associated command queue for receiving input/output command requests from one or more hosts connected to the disk drive system. A "queue", in general, is nothing more than a multi-element data structure from which elements (e.g., command requests) can be removed in the same order in which they were inserted; that is, the queue follows a first-in-first-out (FIFO) constraint. Thus, input/output command requests are typically received in the command queue and executed in the order in which they are received. A command queue typically has a finite maximum length (i.e., predetermined number of records) so that when the queue is full, a host is not able to make a command request until a previously stored command request has been transferred and executed. Hosts of different types respond differently to a "queue full" message. For example, one type of host will simply wait and periodically poll the particular logical volume with its request. Another type of host, upon receiving a "queue full" message will change its mode of operation, for example, to limit the number of requests made in a give period of time. To return to its original mode of operation may require resetting the host (e.g., reboot).